This invention relates generally to means for testing high-power laser mirrors, and, in particular relates to an apparatus for measuring mirror reflectivity of areas on the order of 0.01 mm.sup.2.
Mirrors used in high-power laser applications must have high reflectivity in order to avoid excessive absorption of incident radiation. Heat transfer analysis indicates that, under some critical conditions, the area of small mirror imperfections may grow during laser operation, leading to excessive heating and destruction of the mirror. The critical conditions for the initiation of this phenomenon (known as thermal runaway) depend on the geometric scale of the surface imperfection, its absorptivity, the incident radiation intensity, and the surface temperature level at which the mirror coating deteriorates. Since the latter two parameters are fixed, it is important to characterize coating imperfections with regard to geometric scale and reflectivity, prior to the use of the mirror in an actual laser. High reflectivities are generally achieved by the use of thin multilayer optical coatings of dielectric over a metal coating. The achievement of the desired reflectivity is usually confirmed, prior to a high-power laser test, by the use of a reflectometer. Conventional reflectometers provide average values of reflectivity over relatively large areas on the order of 1 cm.sup.2, yet visual inspection of coated mirrors sometimes indicates small-scale surface imperfections from, 0.01 to 0.1 mm.sup.2 in area. If a laser mirror has a diameter of about a foot, the number of measurements using a conventional reflectometer that inspects an area of about 1 cm.sup.2, is on the order of 500 to 1000. For a microreflectometer that inspects 0.01 mm.sup.2, it takes 10,000 measurements to scan a 1 cm.sup.2 mirror area. Using a manual system of scanning a mirror surface would clearly be a very time consuming process.
These drawbacks have motivated a search for an improved reflectometer system.